The invention relates to metal/air fuel cells, and particularly to a hydrogen removal system for such fuel cells having recirculating electrolyte.
Metal/air fuel cells or batteries produce electricity by the electro-chemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having opposite surfaces respectively exposed to air and to the aqueous electrolyte of the cell. During cell operation, oxygen is reduced within the cathode while metal of the anode is oxidized, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuitry can be connected. Present-day commercial air cathodes are commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) in association with a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, zinc, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight and ability to function as anodes in metal/air fuel cells using a variety of electrolytes.
A typical aluminum/air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface. A typical fuel cell unit or battery comprises a plurality of such cells.
Aqueous electrolytes for metal-air fuel cells consist of two basic types, namely a neutral-pH electrolyte and a highly alkaline electrolyte. The neutral-pH electrolyte usually contains halide salts and, because of its relatively low electrical conductivity and the virtual insolubility of aluminum therein, is used for relatively low power applications. The highly alkaline electrolyte usually consists of NaOH or KOH solution, and yields a higher cell voltage than the neutral electrolyte.
In alkaline electrolytes, the cell discharge reaction may be written:
4Al+30.sub.3 +6H.sub.2 O+4KOH.fwdarw.4Al(OH).sub.4 +K.sup.+ (liquid solution), followed, after the dissolved potassium (or sodium) aluminate exceeds saturation level, by: PA1 4Al(OH).sub.4 +4K.sup.+ .fwdarw.4Al(OH).sub.3 (solid)+4KOH PA1 2Al+6H.sub.2 O.fwdarw.2Al(OH).sub.3 +3H.sub.2 (gas)
In addition to the above oxygen-reducing reactions, there is also an undesirable, non-beneficial reaction of aluminum in both types of electrolyte to form hydrogen, as follows:
When a metal/air fuel cell is operated, quantities of hydrogen form from the surface of the anode. As with other fuel cells or batteries this hydrogen can reach explosive concentrations.
Metal/air fuel cells are of particular interest as a fuel source for motorized vehicles and when they are used for this purpose it is important to keep the hydrogen level in the fuel cell, vehicle and vehicle surroundings below the flammable limit of about 4% by volume. Also when the cells are used in vehicles, the low hydrogen level must be achieved without emitting caustic vapour or mist from the system.
A battery intended as an emergency power supply is described in U.S. Pat. No. 4,490,443, issued Dec. 25, 1984. That battery uses a plurality of individual metal air cells with an electrolyte recirculated through the cells by means of a centrifugal or impeller pump. It is capable of operating under high load, but it has a major disadvantage in that it does not provide a satisfactory means for avoiding hydrogen build-up in the electrolyte reservoir.
U.S. Pat. No. 4,994,332 describes another form of metal/air battery with an alkali metal hydroxide electrolyte circulating through battery cells by means of a pump. The electrolyte exiting from the cells is fed to a knock-out vessel for hydrogen removal from the system. The hydrogen is discharged through an exhaust line.
U.S. Pat. No. 4,950,561 describes a metal/air battery with multiple cells which again recognizes the problem of hydrogen generation in the cells. In that design, the hydrogen gas which is generated either flows with the electrolyte or is exhausted through the electrolyte outlet line into an electrolyte reservoir.
It is the object of the present invention to develop a battery system capable of effectively removing hydrogen from a fuel cell and its electrolyte storage tank without emitting caustic vapour or mist from the system.